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100 lines
3.6 KiB
100 lines
3.6 KiB
// Copyright (c) 2010 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_THREADING_THREAD_RESTRICTIONS_H_
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#define BASE_THREADING_THREAD_RESTRICTIONS_H_
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#include "base/basictypes.h"
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namespace base {
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// Certain behavior is disallowed on certain threads. ThreadRestrictions helps
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// enforce these rules. Examples of such rules:
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//
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// * Do not do blocking IO (makes the thread janky)
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// * Do not access Singleton/LazyInstance (may lead to shutdown crashes)
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//
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// Here's more about how the protection works:
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//
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// 1) If a thread should not be allowed to make IO calls, mark it:
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// base::ThreadRestrictions::SetIOAllowed(false);
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// By default, threads *are* allowed to make IO calls.
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// In Chrome browser code, IO calls should be proxied to the File thread.
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//
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// 2) If a function makes a call that will go out to disk, check whether the
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// current thread is allowed:
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// base::ThreadRestrictions::AssertIOAllowed();
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//
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// ThreadRestrictions does nothing in release builds; it is debug-only.
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//
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// Style tip: where should you put AssertIOAllowed checks? It's best
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// if you put them as close to the disk access as possible, at the
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// lowest level. This rule is simple to follow and helps catch all
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// callers. For example, if your function GoDoSomeBlockingDiskCall()
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// only calls other functions in Chrome and not fopen(), you should go
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// add the AssertIOAllowed checks in the helper functions.
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class ThreadRestrictions {
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public:
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// Constructing a ScopedAllowIO temporarily allows IO for the current
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// thread. Doing this is almost certainly always incorrect.
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class ScopedAllowIO {
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public:
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ScopedAllowIO() { previous_value_ = SetIOAllowed(true); }
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~ScopedAllowIO() { SetIOAllowed(previous_value_); }
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private:
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// Whether IO is allowed when the ScopedAllowIO was constructed.
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bool previous_value_;
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DISALLOW_COPY_AND_ASSIGN(ScopedAllowIO);
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};
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// Constructing a ScopedAllowSingleton temporarily allows accessing for the
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// current thread. Doing this is almost always incorrect.
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class ScopedAllowSingleton {
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public:
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ScopedAllowSingleton() { previous_value_ = SetSingletonAllowed(true); }
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~ScopedAllowSingleton() { SetSingletonAllowed(previous_value_); }
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private:
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// Whether singleton use is allowed when the ScopedAllowSingleton was
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// constructed.
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bool previous_value_;
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DISALLOW_COPY_AND_ASSIGN(ScopedAllowSingleton);
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};
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#ifndef NDEBUG
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// Set whether the current thread to make IO calls.
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// Threads start out in the *allowed* state.
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// Returns the previous value.
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static bool SetIOAllowed(bool allowed);
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// Check whether the current thread is allowed to make IO calls,
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// and DCHECK if not. See the block comment above the class for
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// a discussion of where to add these checks.
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static void AssertIOAllowed();
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// Set whether the current thread can use singletons. Returns the previous
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// value.
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static bool SetSingletonAllowed(bool allowed);
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// Check whether the current thread is allowed to use singletons (Singleton /
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// LazyInstance). DCHECKs if not.
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static void AssertSingletonAllowed();
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#else
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// In Release builds, inline the empty definitions of these functions so
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// that they can be compiled out.
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static bool SetIOAllowed(bool allowed) { return true; }
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static void AssertIOAllowed() {}
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static bool SetSingletonAllowed(bool allowed) { return true; }
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static void AssertSingletonAllowed() {}
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#endif
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private:
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DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadRestrictions);
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};
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} // namespace base
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#endif // BASE_THREADING_THREAD_RESTRICTIONS_H_
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